From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 926F8346AC6; Wed, 18 Mar 2026 19:48:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773863327; cv=none; b=OvgaX1zKU/DizYioByrKRwjS67Qz3asjntj+XoOs9E72oc8SphmON7/LGCMlNBeOkHDH2cpXoAWdGrJJsF/Nvq2KlbbZd+2cow1vhU2iBEdscARODbeBBsd0B1igmTeRqHrWhAGs2qRy4VIkTDXoTICLJDGQVNlfwY1Tz+EWDXk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773863327; c=relaxed/simple; bh=ro1ebNHdPGXGwrEPD80l5QdBY7tZbfdbxOa/h+jJXaw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VlNCFh76FlrtV/lUOXt7lOSSNvdAJwUjjpCSp1aYro1rJFideBPC1s6nOaJrwcsrX699xKBGf6Nl9p2HcMW3VeRYw/aVxqar46lau45UXb1D07g1BQ9VlxrpDJhlvrUsXuGsMN3qfA464rfDtRx+VL6w9x1mHpR5SRDIw4etKj4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Dv40bhex; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Dv40bhex" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9C42BC19421; Wed, 18 Mar 2026 19:48:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773863327; bh=ro1ebNHdPGXGwrEPD80l5QdBY7tZbfdbxOa/h+jJXaw=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Dv40bhexuQMuZDLtQm7he+mWmyxt193jYRqlC6QBz2puRXJzfLOClDBMF03eOc0UG F2CSjew52zK9+MNmUNjFrro4uN/L6kzCnWcbwr93swceQTuKypMchzHD61opvYq1y3 AsSg9CM9rpAN/dHK6e/EkqB77RTVA9qK3h0WBUfh3IEy5myp6AEIlB3MMROVCrA71x 1uT/WWFOkbtjRtN8Eu6oouvUoQmFOK5YjhF0oujTFJm2iAUkMhnkGizS3f8HI0OV1M HlDzgTt2SxWLnKuloDAzgwJQJSygqEcRnbP1QkaLkJv7pqj+d4B3DDynmCJntAe8XG E7MP8Kj4SwThA== Message-ID: <4774da78-8349-4eda-a09b-7248e82cb26b@kernel.org> Date: Wed, 18 Mar 2026 20:48:30 +0100 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v15 11/13] mm/khugepaged: avoid unnecessary mTHP collapse attempts To: Nico Pache , "Lorenzo Stoakes (Oracle)" Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com References: <20260226031741.230674-1-npache@redhat.com> <20260226032631.234234-1-npache@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 3/18/26 19:59, Nico Pache wrote: > > > On 3/17/26 4:35 AM, Lorenzo Stoakes (Oracle) wrote: >> On Wed, Feb 25, 2026 at 08:26:31PM -0700, Nico Pache wrote: >>> There are cases where, if an attempted collapse fails, all subsequent >>> orders are guaranteed to also fail. Avoid these collapse attempts by >>> bailing out early. >>> >>> Signed-off-by: Nico Pache >> >> With David's concern addressed: >> >> Reviewed-by: Lorenzo Stoakes (Oracle) >> >>> --- >>> mm/khugepaged.c | 35 ++++++++++++++++++++++++++++++++++- >>> 1 file changed, 34 insertions(+), 1 deletion(-) >>> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index 1c3711ed4513..388d3f2537e2 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -1492,9 +1492,42 @@ static int mthp_collapse(struct mm_struct *mm, unsigned long address, >>> ret = collapse_huge_page(mm, collapse_address, referenced, >>> unmapped, cc, mmap_locked, >>> order); >>> - if (ret == SCAN_SUCCEED) { >>> + >>> + switch (ret) { >>> + /* Cases were we continue to next collapse candidate */ >>> + case SCAN_SUCCEED: >>> collapsed += nr_pte_entries; >>> + fallthrough; >>> + case SCAN_PTE_MAPPED_HUGEPAGE: >>> continue; >>> + /* Cases were lower orders might still succeed */ >>> + case SCAN_LACK_REFERENCED_PAGE: >>> + case SCAN_EXCEED_NONE_PTE: >>> + case SCAN_EXCEED_SWAP_PTE: >>> + case SCAN_EXCEED_SHARED_PTE: >>> + case SCAN_PAGE_LOCK: >>> + case SCAN_PAGE_COUNT: >>> + case SCAN_PAGE_LRU: >>> + case SCAN_PAGE_NULL: >>> + case SCAN_DEL_PAGE_LRU: >>> + case SCAN_PTE_NON_PRESENT: >>> + case SCAN_PTE_UFFD_WP: >>> + case SCAN_ALLOC_HUGE_PAGE_FAIL: >>> + goto next_order; >>> + /* Cases were no further collapse is possible */ >>> + case SCAN_CGROUP_CHARGE_FAIL: >>> + case SCAN_COPY_MC: >>> + case SCAN_ADDRESS_RANGE: >>> + case SCAN_NO_PTE_TABLE: >>> + case SCAN_ANY_PROCESS: >>> + case SCAN_VMA_NULL: >>> + case SCAN_VMA_CHECK: >>> + case SCAN_SCAN_ABORT: >>> + case SCAN_PAGE_ANON: >>> + case SCAN_PMD_MAPPED: >>> + case SCAN_FAIL: >>> + default: >> >> Agree with david, let's spell them out please :) > > I believe David is arguing for the opposite. To drop all these spelt out cases > and just leave the default case. > > @david is that correct or did I misunderstand that. Either spell all out (no default) OR add a default. I prefer to just ... use the default :) -- Cheers, David